The Millennium Cohort Study is an ongoing longitudinal cohort study headquartered at the Naval Health Research Center in San Diego, California and designed to evaluate any long-term health effects of military service, including deployments.[1] It is the largest population-based prospective health project in US military history,[2][3][4] currently collecting data on over 150,000 enrolled participants. Investigators that conduct the Millennium Cohort Study include uniformed and non-uniformed scientists from the Army,[5] Navy,[6] Air Force,[7]Department of Veterans Affairs,[8] and academic institutions.[9][10]

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After the 1991 Gulf War, the United States Department of Defense recognized the need to collect prospective exposure and health information that may be associated with the long-term health of service members.[11][12] The Millennium Cohort Study was designed to address that need. Pilot studies were conducted in 2000; by mid 2001, the Millennium Cohort Study's first enrollment period was launched, collecting baseline data from over 77,000 people.

Funded by the US Department of Defense,[13] and supported by military, United States Department of Veterans Affairs, and civilian researchers, over 150,000 military personnel are members of the cohort. The Millennium Cohort Study began with a random sample of US Military members including active duty, Reserve, and National Guard members from all services. Surveys are sent to this representative sample of US military personnel every three years through email and the United States Postal Service, requesting that they submit their data online or via the mail service. Approximately 25% of Millennium Cohort participants have left military service and the study will continue to follow all participants through their Active duty, Reserve, National Guard careers and civilian endeavors.

Prospective data analyses are underway to assess health outcomes including Posttraumatic stress disorder,[14][15] depression,[16]hypertension, respiratory symptoms and illness,[17] immune responses, chronic multi-symptom illness, CHD and CVD, and modifiable health behaviors such as smoking, alcohol use[18] sleep,[19] and physical activity that may be associated with deployment in support of the current wars.[20] Currently, more than 50% of Millennium Cohort participants have deployed in support of the wars in Iraq and Afghanistan.

In 2011 the Millennium Cohort Study was expanded to include 10,000 spouses of Millennium Cohort members.
The sub study, The Millennium Cohort Family Study's [21][22] goal is to gain a more complete understanding of the military experience and its resultant impact on the health and well-being of service members and their families.

(2007). Millennium Cohort: The 2001–2003 baseline prevalence of mental disorders in the U.S. Military. Journal of Clinical Epidemiology60 (2): 192–201.

(2007). The physical and mental health of a large military cohort: Baseline functional health status of the Millennium Cohort. BMC Public Health7.

(2008). New onset and persistent symptoms of post-traumatic stress disorder self reported after deployment and combat exposures: Prospective population based US military cohort study. BMJ336 (7640): 366–71.